1. Field of the Invention
The present invention relates generally to the field of transmission and control of optical signals. More particularly, the invention pertains to an electrically controlled apparatus for high speed switching of optical signals. The switching apparatus is especially suitable for applications where the optical signals are transmitted by single mode or multimode optical fibers.
2. Discussion of the Related Art
Until recently, fast optical switching for multimode fiber optic communication and sensor networks was not available. Prior art opto-mechanical switches are known in which the free end of an optical fiber is movable from one contact to another. At each contact, the movable end is aligned with the fixed end of another optical fiber. Such opto-mechanical switches operate at slow speeds (on the order of a few milliseconds) which severely limits their utility. Moreover, repeated flexing of the movable free end of the optical fiber tends to induce fatigue and ultimately results in breakage of the fiber.
By contrast, a fully optical switch (i.e., a switch which uses optical principles for switching as opposed to mechanical principles) is not subject to fatigue and breakage and can operate at much faster speeds.
optical switches have other advantages as well. For example, in environments where electronically switched systems have proved to be unreliable due to electromagnetic interference, systems which use optical switches are able to operate reliably because the optical signals transmitted along the optical fibers are not subject to electromagnetic interference. Moreover, optical switches are more directly compatible with optical fiber networks than electronic switches because optical fibers can be directly connected to optical switches without requiring converters to change between optical signals and electrical signals.
One example of a recently developed, fully optical switch may be seen in U.S. Pat. No. 4,919,522 to Nelson, granted Apr. 24, 1990, and assigned to the same assignee as the present invention, the specification of which is incorporated herein by reference (hereinafter the '522 optical switch). The '522 optical switch uses an electro-optic crystal of the kind having two independent sets of fast and slow optical axes in the crystal. One set of fast and slow optical axes is sensitive to a first electric field extending through the crystal in one direction along a crystal direction and the other set of fast and slow optical axes is angularly disposed with respect to the first set and is sensitive to a second electric field extending through the crystal in a direction orthogonal to the first electric field.
That '522 optical switch also uses a rectangular parallelepiped crystal with electrodes formed on the four opposing sides of the crystal. The activation of one opposed pair of electrodes causes light to pass through one set of optical axes, while activation of the other pair of opposed electrodes causes light to pass exclusively through the other set of optical axes in the crystal.
Although the '522 optical switch operates well as a fast optical switch, it requires a crystal having two sets of well-defined fast and slow optical axes in the crystal or two separate crystals each having a single light path wherein the two light paths must be synchronously switched to achieve a single pole, double-throw switch. Additionally, although the '522 optical switch is optically reversible (or bidirectional), meaning that the switch inputs and outputs can be reversed and the switch will still function properly, the switch has an inherent loss of one-half of the input light intensity due to the polarizing optics required. This insertion loss reduces the overall light transmission through the switch which decreases the number of switches that may be cascaded in a system.
Another example of a recently developed, fully optical switch may be seen in U.S. Pat. No. 5,090,824 to Nelson et al., granted Feb. 25, 1992, and assigned to the same assignee as the present invention, the specification of which is incorporated herein by reference (hereinafter the '824 optical switch). The '824 optical switch achieves a single-pole, single-throw optical switch using a crystal having one set of fast and slow optical axes, one pair of electrodes, one optical path through the crystal, and one set of polarizing optics for beam switching. The '824 switch thus achieves the same result as the '522 optical switch, but uses a single optical path in combination with different polarizing optics that operate in a manner different from the '522 optics resulting in a more compact, less complex switch construction compared to the '522 optical switch.
The '824 optical switch includes a single beam splitting polarizer which detects the change in polarization of a light beam passing through the electro-optic crystal in response to an electric field created in the electro-optic crystal by activation of the pair of electrodes. The beam splitting polarizer transmits light beams received from the electro-optic crystal and having different planes of polarization in different directions. In the '824 optical switch, polarization control optics including a beam splitting polarizer, a right angle prism, and a polarization rotator are used at the input to the electro-optic crystal to reduce the inherent light loss of the switch.
Although the '824 optical switch operates well as a fast optical switch, it requires complex optics to reduce the light loss of the switch. Additionally, the switch is not optically reversible if the polarization control optics are used. If the polarization control optics are not used, then the '824 optical switch would be optically reversible, but would suffer the same loss of one half of the input light intensity as with the '522 switch.